Seals are used in aircraft engines to isolate a fluid from one or more areas/regions of the engine. For example, seals control various parameters (e.g., temperature, pressure) within the areas/regions of the engine and ensure proper/efficient engine operation and stability.
Referring to FIGS. 2A-2B, a prior art sealing system 200 is shown. The system 200 is used to provide an interface between a static engine structure 206 and a rotating engine structure 212. The system 200 includes a floating, non-contact seal 218 that is formed from beams 230a and 230b and a shoe 236 coupled to the beams 230a and 230b. The seal 218 may interface to the structure 206 via a carrier 242. A spacer 248 may separate the carrier 242 and/or the beams 230a and 230b from a seal cover 254. Secondary seals 260 may be included in a cavity formed between the spacer 248, the cover 254 and the shoe 236. The spacer 248 and/or the seal cover 254 may help to maintain an (axial) position of the secondary seals 260. The shoe 236 may interface to (e.g., may slide or rotate with respect to) a scalloped plate 266. The shoe 236 and the beams 230a and 230b may interface to an outer ring structure 288. The seal 218 may include at least some characteristics that are common with a HALO® seal provided by, e.g., Advanced Technologies Group, Inc. of Stuart, Fla.
In operation, air flows from a high pressure area/region 270 of the engine to a low pressure area/region 280 of the engine as shown via the arrow 284. As the air flows passes teeth 238 of the shoe 236 (where the teeth 238 are frequently formed as thin knife-edges), an associated pressure field changes. This change induces the shoe 236 to move in, e.g., the radial reference direction until an equilibrium condition is obtained. In this respect, the seal 218 is adaptive to changing parameters and allows for maintenance of clearances between the static engine structure 206 and the rotating engine structure 212 within a relatively tight range in order to promote engine performance/efficiency. The secondary seals 260 may promote the flow 284 from the high pressure region 270 to the low pressure region 280 as shown between the shoe 236 (e.g., teeth 238) and the rotating structure 212.
In order to accommodate the movement of the shoe 236 described above, the beams 230a and 230b must meet certain structural criteria. On one hand, the beams 230a and 230b must satisfy the natural frequency limit of the rotating structure 212 and must usually be stiffer than a specified threshold, thereby causing the beams 230a and 230b to have a (radial) thickness that is greater than a first (e.g., minimum) threshold. On the other hand, the beams 230a and 230b must be compliant enough to avoid over-stressing them (frequently referred to as having a low cycle fatigue (LCF)) during relative movements/deflections between the structures 206 and 212, thereby causing the beams 230a and 230b to have a (radial) thickness that is less than a second (e.g., maximum) threshold. In many instances, it is difficult to simultaneously satisfy these two competing criteria.